1.\" $NetBSD: explicit_memset.3,v 1.2 2013/08/28 15:46:23 riastradh Exp $ 2.\" 3.\" Copyright (c) 2013 The NetBSD Foundation, Inc. 4.\" All rights reserved. 5.\" 6.\" This documentation is derived from text contributed to The NetBSD 7.\" Foundation by Taylor R. Campbell. 8.\" 9.\" Redistribution and use in source and binary forms, with or without 10.\" modification, are permitted provided that the following conditions 11.\" are met: 12.\" 1. Redistributions of source code must retain the above copyright 13.\" notice, this list of conditions and the following disclaimer. 14.\" 2. Redistributions in binary form must reproduce the above copyright 15.\" notice, this list of conditions and the following disclaimer in the 16.\" documentation and/or other materials provided with the distribution. 17.\" 18.\" THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS 19.\" ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED 20.\" TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR 21.\" PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE FOUNDATION OR CONTRIBUTORS 22.\" BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR 23.\" CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF 24.\" SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS 25.\" INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN 26.\" CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) 27.\" ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE 28.\" POSSIBILITY OF SUCH DAMAGE. 29.\" 30.Dd August 28, 2013 31.Dt EXPLICIT_MEMSET 3 32.Os 33.Sh NAME 34.Nm explicit_memset 35.Nd guarantee writing a byte to a byte string 36.Sh LIBRARY 37.Lb libc 38.Sh SYNOPSIS 39.In string.h 40.Ft void * 41.Fn explicit_memset "void *b" "int c" "size_t len" 42.Sh DESCRIPTION 43The 44.Fn explicit_memset 45function writes 46.Fa len 47bytes of value 48.Fa c 49(converted to an unsigned char) to the string 50.Fa b . 51It is guaranteed not to be optimized away by the compiler even if 52.Fa b 53is no longer used and is about to be freed or go out of scope. 54.Sh RETURN VALUES 55The 56.Fn explicit_memset 57function returns the original value of 58.Fa b . 59.Sh EXAMPLES 60Create a buffer on the stack for a secret key, use it, and then zero it 61in memory before throwing it away. 62.Bd -literal -offset indent 63void 64f(void) 65{ 66 uint8_t key[32]; 67 68 crypto_random(key, sizeof(key)); 69 do_crypto_stuff(key, sizeof(key)); 70 \&... 71 72 explicit_memset(key, 0, sizeof(key)); 73} 74.Ed 75.Sh SEE ALSO 76.Xr consttime_memequal 3 , 77.Xr memset 3 78.Sh HISTORY 79The 80.Fn explicit_memset 81function appeared in 82.Nx 7.0 . 83